218 



INTRODUCTION TO CYTOLOGY 



"end-knob" at the posterior end of the middle piece, while the other passes 

 backward along the axial filament. In mammals (rat, man) Meves 

 (1898, 1899) found much the same situation: the proximal centriole 

 remains near the nucleus and does not elongate to form a middle piece. 

 The distal centriole then divides, one part forming the end-knob and the 

 other passing back as a ring to the end of the "connecting piece" (which 

 does not correspond to the middle piece of the salamander). In (Ecanthus 

 nigricornis, a cricket, the distal centriole, to which the axial filament is 

 attached, constricts off a portion which encircles the filament and works 

 backward along it to the end of the tail sheath (H. Johnson, 1922, 1931). 



WSSm^ 



Fig. 132. — Transformation of nebenkern in Brochymena. {After Bowen, 1922cd.) 



Chondriosomes. — In insects generally the chondriosomes form a single 

 more or less compact body, the nebenkern (the mitosome of Gatenby) 

 (Fig. 132). Globules of a lightly staining fluid appear in the nebenkern, 

 chiefly near its periphery, giving the "blackberry stage" reported after 

 certain fixatives. Soon the more deeply staining portion takes the form 

 of a series of concentric plates alternating with layers of the lighter sub- 

 stance (the "onion stage"). The platework now becomes a hollow ovoid 

 mass with a median partition. As the nebenkern elongates, the ovoid 

 mass shrinks; as it does so, a furrow follows it inward and eventually 

 cuts the nebenkern in two as the chromatic mass vanishes. Meanwhile 

 globules of an additional "central substance" appear in the light neben- 

 kern material, arrange themselves in rows, and coalesce to form beaded 

 strands. As the nebenkern halves elongate further, these strands 

 unite until each half finally has a single axial core composed of the central 

 substance. 



